1. Field of the Invention
The invention relates generally to a dot line printer, and more particularly to an improved print hammer assembly for a dot line printer. The dot line printer has a hammer bank accommodating a plurality of hammers juxtaposed along a print line extending in a direction perpendicular to a direction in which a print paper is fed, wherein printing is carried out during reciprocal movements of the hammer bank along the print line.
2. Description of the Related Art
In a dot line printer, a plurality of print hammers which are magnetically attracted to non-printing positions are selectively fired for making dot impressions on a print paper when a driving current of a predetermined duration is applied to a magnetic coil provided in association with the print hammer. In such a printer, it is a general practice to selectively displace the print hammer positions by a half-dot or a quarter-dot distance form equi-distant positions in order to reduce the number of simultaneously fired print hammers and to decrease the driving current. The half dot distance is defined by a half of a reciprocal of a number of printable dots per unit length, typically expressed by a unit of dots per inch (dpi). Similarly, the quarter dot distance is defined by a quarter of a reciprocal of a number of printable dots per unit length.
FIGS. 1 and 2 show a half-dot displaced hammer arrangement wherein the print pins 42 are alternately displaced a half-dot distance a( a being about 0.07 mm in the illustrated example) from the equi-distance position spaced apart an equi-distance P.sub.1 from the adjacent print pin 42 (P.sub.1 being about 3.8 m in the illustrated example). With such a displaced arrangement, while the number of simultaneously fired print pins are reduced by one half as compared with an equi-distance hammer arrangement, the plungers 432 of the displaced print hammers are also displaced the same distance.
On the other hand, it has been contemplated to tightly arrange an increased number of print hammers in order to speed up printing. However, if the hammer-to-hammer pitch P.sub.1 is reduced to this end, sufficient magnetic attraction is not obtainable due to the reduction of magnetic flux flowing into the plunger. To obviate such a problem, it has been proposed to configure the plunger into a rectangular shape so as to correspond to the configuration of the plunger receiving surface at the comb yoke and to change the circular hole formed in the front yoke to a rectangular groove.
FIG. 3 shows an example of a print hammer assembly having rectangular plungers. As the print hammers are formed in an increased density, the width of the print hammer is smaller than that of the conventional print hammer. In the example shown in FIG. 3, the hammer-to-hammer pitch P.sub.2 is set to about 2.5 mm. Due to the lean print hammer, the horizontal rigidity is lessened and thus the print hammer is liable to be horizontally deformed if force to this effect is applied to the print hammer. Furthermore, since the print pins 52 are alternately displaced a half-dot distance a (a being about 0.07 mm), the associated plunger positions are also displaced the same distance. As a result, the widths of the comb portions of the front yoke 55 are unequal at leftside and rightside of the plunger 53.
To assemble these components, a permanent magnet is magnetized after fixedly securing the leaf spring 51 formed with the print hammers at given pitch. At this time, the magnetic flux flowing into left and right side comb portions of the front yoke 55 through the plunger 53 are unequal. The print pin 52 tends to be inclined toward the wider width comb portion of the front yoke 55, resulting in a horizontal displacement of the print pin 52. If the print hammer is fired in such a condition, the dot impression made on the print sheet is also displaced from the regular position, thus the print quality is degraded.